CODEWITHSUNDEEP
cpointersmemory
Jary316
Jary316

Reputation: 321

C passing pointer to pointer of char

I have a function that gets a char** str_array as such:

char** str_array = Init();

Init allocates an array of char*, which insist contains an array of char.

Then later on I want to free the memory, but I wanted to have a cleanup function, as such:

void FreeStr(char*** arg) {
  char** str = *arg;
  for (int i = 0; str[i] != '\0'; ++i) {
    printf("Str %d: %s\n", i, str[i]);
    free(str[i]);
  }
  free(str);
}

This prints garbage and crashes. The inside of the for loop does work when it is in the same function as where str_array is declared, but not when called as such:

FreeStr(&str_array);

I am confused why this is not working. I thought the extra layer of pointer would make sure I still point to the right memory. What am I misunderstanding please?

Upvotes: 0

Views: 114

Answers (2)

dbush
dbush

Reputation: 223907

There's no need for the extra layer of indirection. Define your function as:

void FreeStr(char** arg)

And call it like this:

FreeStr(str_array);

Also, str[i] != '\0' is not a good way to test for a null pointer. You should be doing str[i] != NULL. So your function should look like:

void FreeStr(char** str) {
  for (int i = 0; str[i] != NULL; ++i) {
    printf("Str %d: %s\n", i, str[i]);
    free(str[i]);
  }
  free(str);
}

Upvotes: 3

Phil Krylov
Phil Krylov

Reputation: 530

You haven't specified if Init() returns a fixed length array or it returns a NULL-terminated array. If the latter is the case, which is the usual practice, the correct code would be:

void FreeStr(char*** arg) {
  char** str = *arg;
  int i;
  for (i = 0; str[i] != NULL; ++i) {
    printf("Str %d: %s\n", i, str[i]);
    free(str[i]);
  }
  free(str);
  *arg = NULL;
}

this way, after calling

FreeStr(&str_array);

provided that str_array is actually a malloc()-allocated NULL-terminated array of malloc()-allocated strings, str_array becomes NULL (you don't want pointers to released memory hanging around, do you?) and all memory is freed.

Upvotes: 4

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